The present invention, in some embodiments thereof, relates to detection of chemicals and, more particularly, but not exclusively, to electrodes, and to systems and methods employing same, usable in electrochemical detection of peroxide-based compounds such as peroxide-based explosives.
An ‘explosive’ is a chemically-unstable molecule having a rapid rate of autodecomposition, with the accompanying evolution of large amounts of heat and gaseous products. There has been a great increase in the development of trace and ultra-trace explosive detection in the last decade, mainly due to the globalization of terrorist acts, and the reclamation of contaminated land previously used for military purposes.
In addition, the availability of raw materials for the preparation of explosives, together with the growing access to information on preparing these explosives, allows for almost anyone with sufficient will and internet access to prepare a bomb. The vast number of people passing through borders, public places, airports etc. poses a huge challenge for current day security screening technologies. The same challenge applies to homes and buildings security. The ultimate goal is to be able to rapidly and effectively screen every passing person, without the need to delay the traffic of people, and without human contact if possible.
Explosives, especially concealed ones, have a very low vapor pressure or ‘signature’ in the surrounding air. The effective vapor pressure of explosives can be reduced by a factor of up to 1000, with the use of plastic packages. Detection methods for traces of explosives therefore continue to be plagued by the low volatility of many target analytes.
One of the most commonly-used high explosives over the last 100 years is 2,4,6-trinitrotoluene (TNT), which poses not only a direct security threat, but also great environmental concern due to soil and water contamination near production, storage and test sites. Other nitro-based explosives are also in use.
Peroxides-based explosives (e.g., cyclic organic peroxides) have also been used recently to build improvised explosive devices, increasing worldwide the awareness thereto. Development of methodologies for the detection of triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD), tetramethylene diperoxide dicarbamide (TMDD) and other cyclic organic peroxides have become an urgent priority. Most organic peroxides are explosive, and some compounds can be easily synthesized by mixing common commercial products such as acetone, hydrogen peroxide and strong acids. Most of the current technology in use for trace detection of explosives is unable to detect peroxide-based explosives [Oxley et al. Propellants, Explosives, Pyrotechnics 34, 539-543 (2009); Önnerud, H., Wallin, S. & Östmark, H. in Intelligence and Security Informatics Conference (EISIC), 2011 European. 238-243 (IEEE)].
Past theoretical studies have showed a plausible approach based on the formation of complexes between the molecular ring structures of cyclic organic peroxide explosives and a central metal moiety, analogous to the formation of clatherates and crown ethers that selectively bind to ionic species in solution. These studies have predicted that TATP molecules can bind to several ions of different valency, with In3+, Zn2+ and Ti4+ showing the highest binding energy [Dubnikova, F., Kosloff, R., Zeiri, Y. & Karpas, Z. The Journal of Physical Chemistry A 106, 4951-4956 (2002)].
Analytical procedures in use today for the trace detection of explosives typically involve collecting vapor samples and analyzing them with a sensitive method. Several methodologies have been reported for detecting TNT and other nitro-based explosives. These are based on electrochemistry, ion-mobility spectrometry, gas chromatography, high-performance liquid chromatography, surface enhanced Raman spectroscopy, nuclear quadrupole resonance, neutron activation analysis, photoluminescence, surface acoustic-wave devices, microcantilevers, fluorescent polymers, surface plasmon resonance, quartz crystal microbalance, immunosensors and other methods. These methods are reliable for explosives detection, but involve time-consuming procedures, high costs and operation by well qualified staff, which limits their application in field conditions.
Additional methods involve trained animals including dogs, mice and bees and utilize their highly sensitive sense of smell for traces explosive detection. These methods however require intense and expensive training of the animal, and handling by an expert.
While a large number of techniques are reported for the detection of nitro based explosives such as TNT, detection of peroxide-based explosives is more complicated. For example, while several detection methodologies rely on the chromophoric nitro groups present in TNT, peroxide-based explosive typically lack chromophoric groups.
Several direct methods for detection of peroxide-based explosives have been reported, including, for example, mass spectrometry, liquid chromatography-mass spectrometry, gas chromatography-mass spectrometry, Ion-mobility spectrometry-mass spectrometry and high-performance liquid chromatography. See, for example, Cotte-Rodriguez et al. Chemical Communications, 953-955, doi:10.1039/b515122h (2006); Widmer et al. Analyst 127, 1627-1632, doi:10.1039/b208350g (2002); Sigman et al. Rapid Communications in Mass Spectrometry 20, 2851-2857, doi:10.1002/rcm.2678 (2006); Buttigieg et al. Forensic Science International 135, 53-59 (2003); and Schulte-Ladbeck, R. & Karst, U. Analytica Chimica Acta 482, 183-188 (2003).
However, these methods suffer from relatively high costs, extensive operator skills, and limited field portability.
A few methods for a rapid detection of peroxide-based explosives based on decomposition of these compounds to hydrogen peroxide in the presence of an acid or ultraviolet radiation and its detection by photometrical and electrochemical methods have also been suggested. See, for example, Schulte-Ladbeck, et al. Analytical Chemistry 75, 731-735, doi:Doi 10.1021/Ac020392n (2003); Schulte-Ladbeck, R. & Karst, U. Analytica Chimica Acta 482, 183-188 (2003); Schulte-Ladbeck et al. Analyst 127, 1152-1154, doi:Doi 10.1039/B206673b (2002); Munoz et al. Analyst 132, 560-565, doi:Doi 10.1039/B701356f (2007); Lu et al. Analyst 131, 1279-1281, doi:Doi 10.1039/B613092e (2006); and Eren, S. et al. Analyst 135, 2085-2091, doi:Doi 10.1039/B925653a (2010).
Electrochemical detection methods utilize electrodes, immersed in an electrolyte, and connected to a potentiometer, which measure the current that flows between the electrodes upon potential application. Typically, during an electrochemical reaction the electrode potential is varied; and an electric current flows between the electrodes that is characteristic of the presence of an electrochemically reactive substance in the electrolyte. Electrochemical detection typically meets most of the above requirements of a robust and efficient methodology for detection of explosives. See, Caygill, J. S., Davis, F. & Higson, S. P. J. Current trends in explosive detection techniques. Talanta 88, 14-29, doi:DOI 10.1016/j.talanta.2011.11.043 (2012); Wang, J. Electrochemical sensing of explosives. Electroanal 19, 415-423, doi:DOI 10.1002/elan.200603748 (2007).
Most electrochemical methods for peroxide-based explosives detection rely on the detection of H2O2 formed from the acid or ultraviolet decomposition of the explosive material. Prussian-blue and FeII/III ethylenediaminetetraacetate are examples of chemical modifications on a working electrode that are used in those methods. Additional examples include electrochemical detection of TATP performed by redox reaction with bromide ion at 55° C. Acetone and bromine, obtained in such a reaction, interact to give acetone bromides, such that a lack of free bromine is indicative of the presence of the explosive. Another example is the detection of HMTD using electro-generated chemiluminescence (ECL), while utilizing the tertiary amine moieties present in HMTD. See, for example, Schulte-Ladbeck et al. Analyst 127, 1152-1154, doi:Doi 10.1039/B206673b (2002); Lu et al. Analyst 131, 1279-1281, doi:Doi 10.1039/B613092e (2006); Laine et al. Analytica Chimica Acta 608, 56-60, doi:DOI 10.1016/j.aca.2007.12.003 (2008); Laine et al. Microchem J 91, 78-81, doi:DOI 10.1016/j.microc.2008.08.005 (2009); Zhao et al. Journal of Electroanalytical Chemistry 379, 501-503, doi:Doi 10.1016/0022-0728(94)87175-2 (1994); Xie, Y. Q. & Cheng, I. F., Microchem J 94, 166-170, doi:DOI 10.1016/j.microc.2009.10.016 (2010); Parajuli, S. & Miao, W. J., Analytical Chemistry 81, 5267-5272, doi:Doi 10.1021/Ac900489a (2009).
One of the most pronounced limitations in electrochemical measurement under atmospheric conditions is the presence of dissolved oxygen in a sample. The dissolved oxygen concentration in aqueous electrochemical solution is about 0.25 mM (about 8 ppm) [Julia, P. L. C. & Coso, E. B. Homenatge professor Josep M. Costa (eBooK) 2a part. Trends in electrochemistry and corrosion at the beginning of the 21st century. (Publicacions i Edicions de la Universitat de Barcelona, 2004)] and is much higher in non-aqueous electrochemical solutions [Achord, J. M. & Hussey, Analytical Chemistry 52, 601-602, (1980)]. The oxygen is reduced practically at the same potentials as peroxides-based explosives, and, since oxygen concentration is higher by several orders of magnitude than that of the explosive traces, signals generated by the peroxide-based explosives traces are substantially masked. This limitation is typically treated by deaeration; the oxygen is removed by means of bubbling inert gas, for example argon or nitrogen. Typically, 10-15 minutes of deaeration are required in order to obtain efficient oxygen removal in a sample of approximately 5 ml. This lengthy procedure is not in line with the requirements for real time detection of nitro-containing explosives [W. Chen, Y. Wang, C. Bruckner, C. M. Li, Y. Lei, Sensor Actuat B-Chem 2010, 147. 191-197].
In addition, a major overlap of the reduction peak of traces of H2O2, which may be found in field conditions, and that of a peroxide-based explosive further complicate the electrochemical detection, often leading to “false positive” detection. See, for example, Butler et al. Talanta 41, 211-215, (1994); Marinović et al. Journal of Electroanalytical Chemistry 648, 1-7, (2010).
WO 2011/154939 describes nanodevices which utilize functionalized nanowires for detecting nitro-containing compounds. The nanowires feature a functional moiety that interacts with a nitro-containing compound by forming a charge-transfer complex therewith.
WO 2005/050157, WO 2006/090401, and WO 2007/029245 teach systems for detecting traces of nitro-aromatic compounds in air, which utilize carbon electrodes modified by amino-aromatic compound or nitrogen-containing heterocyclic compounds.
WO 2014/111944 describes nanodevices which utilize functionalized nanowires for detecting peroxide-based and/or nitro-containing explosives.
Additional background art includes Spalek ET AL. J Chem Soc Farad T 1 78, 2349-2359, doi:Doi 10.1039/F19827802349 (1982); Weiss, J. Transactions of the Faraday Society 31, 1547-1557 (1935); Sheppard, S. A. et al. Analyst 123, 1923-1929, doi:Doi 10.1039/A803310b (1998); and Chaki et al. Chemical Communications, 76-77, doi:Doi 10.1039/B107965b (2002).
Yet additional background art includes Chaubey, A. & Malhotra, B. D. Mediated biosensors. Biosens Bioelectron 17, 441-456, (2002); Zang et al. Analytica Chimica Acta 683, 187-191, (2011); U.S. Pat. No. 6,872,786; Chen et al. Sensor Actuat B-Chem 147, 191-197, (2010); Filanovsky, B. et al. Adv Funct Mater 17, 1487-1492, (2007); Grigoriants, I. et al. Electrochim Acta 54, 690-697, (2008); Guo et al. Electroanal 23, 885-893, (2011); Chen et al. Chemistry—An Asian Journal 6, 1210-1216, (2011); Wang et al. Sensors-Basel 11, 7763-7772, (2011); Cizek, K. et al. Analytica Chimica Acta 661, 117-121, (2010); Galik et al. Electroanal 23, 1193-1204, doi:DOI 10.1002/elan.201000754 (2011); WO 2010/112546; WO 2010/227382; WO 2015/059704; WO 2017/098518; Engel, Y. et al. Angew Chem Int Edit 49, 6830-6835, (2010); Dwivedy et al. Journal of Chromatography A 29, 120-125 (1967); and Lichtenstein, A. et al. Nat Commun 5, (2014).